This invention relates to a pressure sensor with long term stability and high sensitivity useful in various fields of instrumentation including gas analysis and Luft type infrared gas analyzers as well as in photoacoustic and magnetoacoustic analyzer systems.
The task of measuring low levels of gas pressure with long term stability and high sensitivity is important in various field of instrumentation, particularly those related to gas analysis. Several classes of instrumentation rely on inducing small pressure changes in a gas sample by exposing the gas to a controlled stimulation or environment such as exposure to light, a magnetic field or an electric field stimulus. Luft type infrared gas analyzers as well as photoacoustic and magnetoacoustic analyzers, for example, rely on differential pressure measurements at frequencies that may, for example, range from a fraction of hertz to several kilohertz and with low pressure variations down to about 10xe2x88x926 Pa (Pascal) detection thresholds. A pressure sensor capable of detecting such small pressure variations would be extremely useful in these environments. In general, there is a need to make dynamic pressure measurements within about plus or minus 1% accuracy over a long term (months to years), and in temperature ranges of up to several tens of degrees centigrade. In some situations, resistance to chemicals is also required.
By way of example, one specific application is a paramagnetic oxygen sensor. A sample of gas in a cell is subject to a time varying magnetic field that acts upon a paramagnetic component (e.g., oxygen) present in the gas to produce pressure variations that are to be detected by the pressure sensor. The time varying magnetic field produces pressure variations in the range of a fraction of a Hertz to 10 kHz. The amplitude of the signal output by the pressure sensor provides a measure of the oxygen concentration in the sample with typical pressure sensitivity of better then 10xe2x88x925 Pa required for about 10xc2x1ppm oxygen sensitivity. Conventional pressure sensor designs, however, are poorly adapted to detect such low-amplitude pressure changes.
One common design for measuring pressure at near vacuum applications involves a diaphragm typically made of plastic under significant tension adhered to a ring to form a sealed differential pressure sensor assembly. However, the diaphragm tension change due to exposure to chemicals (vapors), thermal stresses in the housing, aging effects, and other sources of drift all have a great influence on the diaphragm displacement under pressure. A relatively significant tension of the membrane is required to reasonably offset these effects, which limits sensitivity. This is especially true when pressure variations down to about 10xe2x88x926 Pa are to be measured. Moreover, in prior art designs, collateral variations in the sensor output necessitates frequent recalibrations or may require operation under elaborate temperature controls, signal offset, or other corrections.
It is therefore an object of this invention to provide a differential pressure sensor that is sufficiently sensitive to be used in measurement instruments such as sensitive gas analyzers and the like.
It is a further object of this invention to provide such a high sensitivity pressure sensor with long term stability.
It is a further object of this invention to provide an accurate differential pressure sensor with a high level of common-mode rejection at frequencies up to several kHz.
It is a further object of this invention to provide a pressure sensor which can be used for measuring low levels of gas pressure and thus useful in ppm range paramagnetic oxygen sensor and other implementations.
It is a further object of this invention to provide such a pressure sensor which is not sensitive to temperature changes or to the presence of corrosive chemical vapors as well as solvents, hydrocarbons, and the like.
It is a further object of this invention to provide such a pressure sensor which does not require frequent recalibrations.
It is a further object of this invention to provide such a pressure sensor which automatically compensates for variations due to slow effects such as dimensional or other (e.g., tensile) changes induced by temperature gradients.
This invention results from the realization that a more accurate, stable, and high sensitivity pressure sensor is effected by the inclusion of a compensation circuit configured to apply an electric field to the solid (e.g., metal), membrane of the sensor thereby creating a force to compensate for variations due to long-term or slow effects such as dimensional or tensile changes, by the use of a housing having a coefficient of thermal expansion the same as or similar to the coefficient of thermal expansion of the thin solid material membrane, and by the incorporation of a symmetrical design so that membrane displacement is associated linearly with voltage.
This invention features a high sensitivity pressure sensor with long term stability comprising a housing including first and second chambers, a membrane separating the first and second chambers, a first electrode located in the first chamber and spaced from one side of the membrane forming a first capacitor therewith, and a second electrode located in the second chamber and spaced from an opposite side of the membrane forming a second capacitor therewith. A measuring circuit is connected across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors and a compensation circuit is configured to apply an electric field to the membrane as a compensating force by reducing the voltage difference between the first electrode and the membrane and simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa to provide long term stability.
Typically, the membrane is made of metal and the metal membrane is not under significant tension in the absence of a differential pressure between the two chambers. In the preferred embodiment, the tension on the membrane is less than 0.36 N/m. Also in the preferred embodiment, membrane displacement versus pressure due to the bending force of the membrane is Ybend, the membrane displacement versus pressure due to tension is Ytens, and Ybend less than  less than Ytens. Preferably, the housing is made of a material having a coefficient of thermal expansion the same as or substantially the same as the coefficient of thermal expansion of the material of the membrane, e.g., the housing is made of titanium and the membrane is made of titanium.
In one example, the housing includes first and second base plates each with an inner chamber and the membrane is disposed between the two base plates separating the inner chambers thereof and the two base plates are secured together under compression and then released to slightly tension the membrane to prevent negative tension thereof. The first electrode may be attached to a first holder affixed to the first base plate over its inner chamber and the second electrode is then attached to a second holder affixed to the second base plate over its inner chamber. Further included may be an insulator between the first holder and the first base plate and an insulator between the second holder and the second base plate. In a typical embodiment, a first seal is disposed about the first electrode sealing it with respect to the first base plate inner chamber and a second seal is disposed about the second electrode sealing it with respect to the second base plate inner chamber. The first base plate may include a conduit in communication with the inner chamber thereof and the second base plate then also includes a conduit in communication with the inner chamber thereof.
In the preferred embodiment, the membrane has a thickness of between 5-15 micrometers, the volume of the first chamber is substantially the same as or the same as the volume of the second chamber, and the spacing between the first electrode and the membrane is the same as or substantially the same as the spacing between the second electrode and the membrane. The spacing may be between 10 and 30 micrometers. Also, the first electrode typically has a substantially planar surface facing the membrane and the second electrode also has a substantially planar surface facing the membrane.
In the preferred embodiment, the compensation circuit includes a voltage divider connected to the first and second electrodes, and an amplifier connected to a middle point of the voltage divider for monitoring the electrical potential of the middle point. Also, the measuring circuit includes a bridge circuit connected to the first and second electrodes and having feedback to the compensation circuit. The bridge circuit may include a high frequency voltage source, namely the secondary coil of a transformer, the primary of which is connected to a high frequency voltage generator. The compensation circuit may further include a lock-in amplifier connected between the amplifier and the measuring circuit, the lock-in amplifier including a phase shifter connected to a high frequency voltage source and a phase detector connected to the amplifier. In one example, a low pass filter is disposed between the output of the phase detector and the amplifier.
An exemplary high sensitivity pressure sensor with long term stability in accordance with this invention features a housing including first and second chambers, a metal membrane separating the first and second chambers, the metal membrane not under significant tension in the absence of a differential pressure between the two chambers, a first electrode located in the first chamber and spaced from one side of the metal membrane forming a first capacitor, a second electrode located in the second chamber and spaced from an opposite side of the metal membrane forming a second capacitor, a measuring circuit for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors, and a compensation circuit including a voltage divider connected to the first and second electrodes and having a middle point which when unbalanced applies a greater electrostatic field to one side of the membrane than the other side of the membrane.
An exemplary control circuit for a differential pressure sensor including first and second capacitors and a membrane common to both capacitors in accordance with this invention includes a measuring circuit connected across the first and second capacitors for measuring membrane displacement by detecting differences in capacitance between the first and second capacitors, and a compensation circuit configured to apply an electrostatic field to the membrane thereby producing a compensating force by reducing the voltage difference between the first electrode and the membrane and by simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa to provide long term stability.
A method of making and using a high sensitivity pressure sensor with long term stability, in accordance with this invention, features forming a housing to include first and second chambers, disposing a membrane to separate the first and second chambers, placing a first electrode in the first chamber spaced from one side of the membrane forming a first capacitor therewith, placing a second electrode in the second chamber spaced from an opposite side of the membrane forming a second capacitor therewith, connecting a high frequency measuring circuit across the first and second capacitors for measuring dynamic membrane displacement by detecting differences in capacitance between the first and second capacitors, and applying an electrostatic field to the membrane generating a compensating force to reduce the voltage difference between the first electrode and the membrane and to simultaneously increase the voltage difference between the second electrode and the membrane or vice versa to provide long term stability.
A method of operating a high sensitivity pressure sensor to have long term stability in accordance with this invention includes measuring membrane displacement by detecting differences in capacitance between the first and second capacitors, and applying an electric field to the membrane as a compensating force by reducing the voltage difference between the first electrode and the membrane and by simultaneously increasing the voltage difference between the second electrode and the membrane or vice versa.